1. Field of the Invention
The invention relates to a data weighted average circuit, which is applied to a data conversion system such as a sigma-delta modulator (SDM) or a digital-to-analog converter (DAC).
2. Description of the Related Art
In general, there are three common approaches to increasing the resolution of SDMs, i.e., reducing the quantization noise within the signal bandwidth. The first approach is to increase the over-sampling ratio (OSR). The second approach is to increase the number of orders of SDMs. The third approach is to increase the resolution of quantizers, i.e., increasing the number of bits. The third approach directly reduces the overall quantization noise to achieve a higher resolution. A multibit quantizer needs to include a corresponding number of bits only to be linear, whereas the linearity of a multibit DAC has to be greater than the integral linearity of the SDM. For example, a 3-order 2-bit SDM with a 14-bit signal-to-noise ratio (SNR) includes a 2-bit DAC with linearity of at least a 14-bit level.
For example, a 3-bit (8-level) DAC includes 7 digital-to-analog converting (D/A converting) cells, each of which includes one capacitor CS. The mismatch between the capacitors CS has to be low enough for achieving a high linearity. However, in practical, it is not realizable since the capacitors CS need to have large capacitance. Thus, previous paper has proposed a method of dynamic selecting capacitors of the DAC cells, which moves the distortion caused by the mismatch out of the signal band. This is called dynamic element matching. Nys et, al, “A 19-Bit Low-Power Multibit Sigma-Delta ADC Based on Data Weighted Averaging,” IEEE Journal of Solid-State Circuits, VOL. 32, No. 7, July 1997, discloses a circuit architecture of a Sigma-Delta ADC. And, Baird et, al, “Linearity Enhancement of Multibit ΔΣA/D and D/A Converters Using Data Weighted Averaging,” IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, VOL. 42, No. 12, December 1995, discloses a method of linearity enhancement.